For your clarification on the CEL. The car throws a CEL error code because the 2nd o2 sensor is not sending a reading because it has been removed. When you have your tune changed the tuner will turn off your second o2 sensor check thus not getting an error sent to your ecu for a fault in the rear o2 sensor because that parameter is turned off and is not being monitored. So they are not turning the CEL code off but turning off the sensor that would cause the error code.